Here is a trick for multiplying a 2-digit number that ends in 1 by another 2-digit number:
Multiply the tens digit of the first number by the tens digit of the second number. For example:
21 * 35 = 7
Multiply the units digit of the first number (1) by the units digit of the second number (5). For example:
1 * 5 = 5
Add the result from step 1 to the result from step 2 and put a 1 at the end. For example:
7 + 5 = 12, so the result is 121.
For example, to find 21 * 35, we multiply the tens digit of the first number (2) by the tens digit of the second number (3) to get 6, multiply the units digit of the first number (1) by the units digit of the second number (5) to get 5, and add the results to get 11. We then put a 1 at the end to get the final result of 121.
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